1. Field of the Invention
This invention relates to an injection apparatus for a hot-chamber die-cast machine, and more particularly, to an injection apparatus having a new and improved injection plunger which can be easily designed and produced and is useful to overcome various problems, such as leakage of molten metal or the like, in practical use.
2. Description of the Related Art
An injection apparatus for a hot-chamber die-cast machine for casting molten metal, such as aluminum or a similar metal, has been conventionally known in the prior art. One known hot-chamber die-cast machines for casting aluminum is generally shown in FIG. 10, in which a furnace 1 for melting and accommodating a molten metal 15 includes heaters 3 for heating the furnace 1 up to a certain temperature so as to keep the metal in the furnace to a molten state. The furnace 1 mounted on a base frame 2 is provided with a goose-neck 13 having a nozzle 14 which is forced and fastened to a mold 4 attached to a die-plate 5.
In order to rigidly connect the mold 4 to the nozzle 14, the base frame 2 is clamped to the die-plate 5 by a clamping means (not shown). Above the furnace, in a sleeve 10, is inserted a plunger 8 which is connected to a hydraulic piston 6 by means of a coupling 7, so that when the piston 6 moves downward, the plunger 20 is also moved downward to pressurize the molten metal in the sleeve 10. Thus, the molten metal flows through the goose-neck 13 and is injected via the nozzle 14 into the cavity of the mold 4 and a casting operation is thus performed.
Japanese Unexamined Patent Publications (kokai) Nos. 56-23358 and 56-23359 discloses a plunger used in the above-mentioned injection apparatus, in which the plunger 8 is a simple cylindrical type plunger, as shown in FIG. 11, and therefore has no pressure ring or the like. However, such a simple cylindrical type plunger having no pressure ring must be very accurately made in order to obtain a stable sealing effect between the outer diameter of the cylindrical type plunger and the inner diameter of the sleeve. Thus, the clearance between the cylindrical plunger and the sleeve must strictly be controlled to be less than 0.04 mm. Nevertheless, since the material of the cylindrical plunger is a ceramic, it is very difficult to obtain a precise dimension over the entire surface of the plunger. Also, a cost for making such a ceramic plunger is thus relatively expensive.
When assembling the above-mentioned plunger into the sleeve, considerable skill is required to position the sleeve with respect to the plunger and, therefore, careful attention and laborious work are required. In addition, during the operation of the same plunger, a blockage might cause thermal deformation or a slight offset of the plunger and, thus, the effective life of the plunger or the sleeve might greatly be reduced.
If the molten metal has a low corrosion characteristic, a pressure ring 9' can be attached to the plunger 8, as shown in FIG. 10. Such a pressure ring 9' is springingly urged to the inner surface of the sleeve 10 when the plunger 8 is moved to obtain a sealing effect between the plunger 9' and the sleeve 10. Thus, such a pressure ring 9' is usually made of any suitable spring material, such as a steel or the like, to attain a required spring operation.
Japanese Unexamined Patent Publication (kokai) No. 51-31628 and Examined Utility Model Publication (kokoku) No. 47-16574 disclose related prior art in which such a plunger has a pressure ring or rings.
However, in a plunger for injecting an aluminum or the like, since most metals are usually corroded by molten aluminum, a metal plunger cannot be used, in practice, for the purpose. In order to overcome such a disadvantage, a plunger having divided rings made of a ceramic has been proposed in Japanese Unexamined Patent Publication (kokai) No. 56-23360.
In the known plunger as mentioned above, however, the ceramic divided ring must be very precisely made with the inner surface of the sleeve to obtain a stable sealing effect between the ring and the sleeve, in the same manner as the cylindrical ceramic plunger. It is very difficult to obtain precise clearance between the ring and the sleeve and, therefore, considerable skill and expense are required.